The present invention relates to cylinder apparatus such as hydraulic shock absorbers.
A cylinder type hydraulic shock absorber as a cylinder apparatus attached, to a suspension system of a vehicle, e.g. an automobile, generally has a cylinder having a fluid sealed therein, a piston slidably fitted in the cylinder, a piston rod connected to the piston, and a damping force control mechanism comprising an orifice, a disk valve, etc. In response to a stroke of the piston rod, the piston slidingly moves in the cylinder, causing a flow of fluid. The fluid flow is controlled by the damping force control mechanism, thereby generating a damping force.
Japanese Patent Application Laid-open Publication No. 2006-38097 discloses a hydraulic shock absorber in which a back-pressure chamber (pilot chamber) is formed at the back of a disk valve constituting a damping force generating mechanism, and a part of the flow of fluid is introduced into the back-pressure chamber. The pressure in the back-pressure chamber is applied to the disk valve in the direction for closing the disk valve. The pressure in the back-pressure chamber is adjusted with a pilot valve to control the valve-opening operation of the disk valve. This arrangement makes it possible to increase the degree of freedom for adjusting damping force characteristics.
In the shock absorber disclosed in Japanese Patent Application Laid-Open Publication No. 2006-38097, a ring-shaped seal member made of an elastic material, e.g. rubber, is fixed to the outer periphery of the rear side of the disk valve by vulcanization bonding or the like. The seal member is slidably and fluid-tightly fitted to a cylindrical portion of a bottomed cylindrical member which is disposed at the back of the disk valve, thereby forming a back-pressure chamber.
With a disk valve having a ring-shaped seal member fixed to one side thereof as stated above, the seal member may contract in a manufacturing process where the seal member is fixed to a disk valve body. The contraction of the seal member may cause the disk valve body to be slightly deflected, resulting in degradation in flatness of the disk valve.
If the flatness of the disk valve is degraded, the disk valve cannot come in sufficiently close contact with the associated valve seat, so that the hydraulic fluid may leak, resulting in unstable damping force characteristics. Accordingly, conventional practice is to apply a set load (preload) to the disk valve when installed, i.e. to press the disk valve against the valve seat so as to slightly deflect the disk valve, thereby ensuring close contact between the disk valve and the valve seat. With, the conventional practice, however, the set load causes an increase in the valve-opening pressure of the disk valve; therefore, it becomes difficult to obtain a sufficiently small damping force in a low piston speed region. In recent years, a damping force control type shock absorber installed in a suspension system of a vehicle, e.g. an automobile, has been demanded to generate a sufficiently small damping force in a low piston speed region when the damping force characteristics are set to the “soft” side. In this regard, applying a set load, to the disk valve is contrary to the above-described demand.